Steering control apparatus and method

ABSTRACT

A vehicle steering control device is provided that suppresses changes in the steering force accompanying the shock transmitted from the road surface during the steering wheel return operation. The vehicle steering device is a steer-by-wire steering device where a steering wheel receiving the steering input which is transmitted electronically to the steering unit which turns steered road wheels of the vehicle. The steering reaction force correction (Gf×F) corresponding to road surface reaction force F is applied to the steering wheel. The device includes a turn/return sensor that senses turn/return of the steering wheel, and during return of steering wheel, road surface reaction force feedback gain Gf is made smaller than the initial turning.

BACKGROUND

The present invention relates to the field of steering control forvehicles and in particular to electronic steering control systems.

In a conventional steer-by-wire system as described in Japanese KokaiPatent Application No. Hei 10[1998]-217988, the steering reaction forcecorrection proportional to the detected road surface reaction force iscomputed and added to the steering reaction force, so that the conditionof the road surface is transmitted to the driver.

However, in the vehicle steering device of the prior art, for example,when the vehicle turns an L-shaped corner so that a quick steering wheelresponse is required, or a shock from the road surface is transmitteddue to a bumpy road surface as the steering wheel is turned back, atransient steering force occurs abruptly, the steering wheel may hinderthe turning back of the steering wheel, which is undesirable.

In the vehicle steering device described in Japanese Kokai PatentApplication No. Hei 10[1998]-217988, and particularly in the steeringforce computation unit, on the basis of the detection result of thesteering force sensor, a steering force (T) applied to the steeringcolumn (steering shaft) is computed. At the same time, a control value(aT) (where a is the coefficient corresponding to the steering forcegear ratio) for rotating the steering shaft in the direction of appliedsteering force (T) is also computed.

The road surface reaction force from the steering reaction force sensoris transmitted to the steering reaction force, for example, when thevehicle turns an L-shaped corner, so that quick return steering isrequired. If the tire dips due to the rough road surface (holes, etc.),due to the signal from the steering reaction force sensor, quickmaneuvering of the steering wheel may be hindered, which is undesirable(see FIG. 7). If the embodiment of the present invention is not applied,this occurs because the steering reaction force is added to track thekickback from the road surface, the steering force rises abruptly, andit becomes difficult for the driver to respond quickly.

SUMMARY

In accordance with one aspect of the invention, a steering controldevice is provided for use in a vehicle having a steering wheel thatreceives steering input, and an electronically-controlled steering unitthat turns the vehicle's wheels over a road surface based on theposition of the steering wheel. The steering control device includes areaction force device coupled to the steering wheel and responsive to acontrol signal to apply a steering reaction force to the steering wheel;and a controller adapted to generate the control signal in response tothe movement of the steering wheel and the road surface reaction force.The controller varies the control signal to increase the steeringreaction force in response to the road surface reaction force when thesteering wheel is turning and to decrease the reaction force in responseto the road surface force when the steering wheel is returning.

In accordance with another aspect of the invention, a method forcontrolling steering in a vehicle having a steering wheel and a reactiondevice to impose a steering reaction force onto the steering wheel inresponse to a steering force control signal. The method includescalculating the steering force control signal based on a road surfacereaction force and a gain; determining whether the steering wheel is ina turning or returning mode; and setting the gain at a higher value whenthe steering wheel is in a turning mode.

BRIEF DESCRIPTION OF THE DRAWINGS

The description herein makes reference to the accompanying drawingswherein like reference numerals refer to like parts throughout theseveral views, and wherein:

FIG. 1 is a schematic system diagram illustrating the vehicle steeringsystem according to the first embodiment.

FIG. 2A is a detailed sectional diagram illustrating the clutch, in thevehicle steering device of the first embodiment.

FIG. 2B is a detailed sectional diagram illustrating the cable column inthe vehicle steering device of the first embodiment.

FIG. 2C is a detailed sectional diagram illustrating the torque sensorin the vehicle steering device of the first embodiment.

FIG. 3 is a flow chart illustrating the method for setting road surfacereaction force feedback gain (Gf) according to the first embodiment.

FIG. 4 is a graph road surface reaction force feedback gain (Gf)corresponding to road surface reaction force (F).

FIG. 5 is a graph of variable L1 corresponding to steering anglevelocity dθ/dt.

FIG. 6 is a graph of variable L2 corresponding to vehicle speed (V).

FIG. 7 is a graph of steering reaction force corresponding to steeringangle in the turn and return operation of a steering wheel.

FIG. 8 is a graph of steering reaction force corresponding to steeringangle in the turn and return operation of a steering wheel in accordancewith the first embodiment of the invention.

FIG. 9 is a graph shows the graph used to set (YD).

FIG. 10 shows the graph used to set (Gy).

FIG. 11 is a graph of steering reaction force corresponding to steeringangle in the turn and return operation of a steering wheel in accordancewith the first embodiment of the invention.

FIG. 12 is a flow chart illustrating the method used to set road surfacereaction force feedback gain (Gf) according to a third embodiment of theinvention.

FIG. 13 is a graph of the rotation limit evaluation corresponding to yawrate ψ and the steering angle.

DETAILED DESCRIPTION

FIG. 1 is an overall system diagram illustrating the vehicle steeringdevice of the first embodiment. FIGS. 2 a-c are detailed diagramsillustrating the clutch, cable column, and torque sensor components,respectively, in the vehicle steering device of the first embodiment.The vehicle steering device of composed of a reaction force device, anauxiliary device, an electronically-controlled steering device, and acontroller.

The reaction force device has steering angle sensor 1, encoder 2, torquesensors 3, and reaction force motor 5.

The steering angle sensor 1 is a means for detecting the angularposition of steering wheel 6. It is set on a column shaft 8 a thatconnects a cable column 7 and a steering wheel 6. That is, steeringangle sensor 1 is placed between steering wheel 6 and torque sensors 3and is unaffected by the change in angle due to the twisting of torquesensors 3 so that the sensor 1 can detect the steering angle. In thesteering angle sensor 1, an absolute type resolver (not shown) or thelike is used.

The torque sensors 3 form a double system and are arranged between thesteering angle sensor 1 and reaction force motor 5. The system is madeup of two torque sensors, that is, torque sensor 3 and torque sensor 12.FIG. 2C is a diagram illustrating in detail a torque sensor unit. Eachtorque sensor 3 has a torsion bar extending in the axial direction, afirst shaft connected to one end of the torsion bar and coaxial to thetorsion bar, a second shaft connected to the other end of the torsionbar and is coaxial to the torsion bar and the first shaft, a firstmagnetic body fixed to the first shaft, a second magnetic body fixed tothe second shaft, a coil facing the first magnetic body and the secondmagnetic body, and a third magnetic body that forms a magnetic circuittogether with the first magnetic body and second magnetic body. The coildetects the torque from the output signal on the basis of the inductancethat changes corresponding to the relative displacement between thefirst magnetic body and the second magnetic body on the basis of thetwisting of the torsion bar.

The reaction force motor 5 is a reaction force actuator that imparts areaction force to the steering wheel 6. The reaction force motor 5 ismade of a 1-rotor/1-stator type of electric motor with the column shaft8 a as the rotary shaft. The housing is fixed at an appropriate locationon the vehicle body. A brushless motor is used as the reaction forcemotor 5 with the encoder 2 and a Hall IC (not shown in the figure),which are required for use with a brushless motor. If only a Hall IC isused, although it will be possible to drive the motor that generates themotor torque, nevertheless there will be small variations in the outputtorque, and the feel of the steering reaction force will be poor. Inorder to effect smoother control of the reaction force, encoder 2 isplaced on the shaft of column shaft 8 a to control the motor. As aresult, the small torque variations can be reduced, and the steeringreaction force feel is improved. As an alternative, a resolver can beused in place of encoder 2.

The auxiliary unit is composed of cable column 7 and clutch 9. Theclutch 9 is arranged between column shaft 8 a and pulley shaft 8 b. Anelectromagnetic clutch is used in the first embodiment. FIG. 2A is adiagram illustrating in detail the clutch component 9. When power isturned on to the electromagnetic clutch, clutch 9 generates magneticflux Φ. In this case, because the armature is magnetically drawn to thebrushes of the rotor against the restoring force of a leaf spring,column shaft 8 a, the input shaft, and pulley shaft 8 b, the outputshaft, are connected to each other. Then, as steering wheel 6 isrotated, its rotational force is transmitted via clutch 9 to the pulleyof cable column 7 to rotate the pulley of cable column 7. As a result,the rotational force is transmitted via clutch 9 to steering wheel 6.Also, when the power is turned off to the electromagnetic coil, magneticflux Φ collapses, and the armature leaves the rotor due to the restoringforce of the leaf spring. That is, the transmission torque capacity ofclutch 9 can be set as desired by changing the drawing force as magneticflux Φ generated by the magnetic coil is changed. Also, a scheme inwhich the clutch is released when the power is turned on may also beused.

The cable column 7 has a mechanical backup mechanism that can play thepart of the column shaft in transmitting the torque while it detours toavoid interference with the element included between the reaction forcedevice and the steering device. FIG. 2B is a diagram illustrating indetail the cable column unit. In the structure of cable column 7, twointerior cables, each end of which is fixed to a reel 22, are wound ontothe two reels 22, and the two ends of the exterior sheath in which twoinner cables are inserted are fixed to two reel housings.

The steering unit includes encoder 10, steering angle sensor 11, torquesensors 12, steering motors 14, steering unit (steered wheel turningunit) 15, and steered wheels 16, 16′.

The steering angle sensor 11 and torque sensors 12 are mounted on pinionshaft 17, on one end of which the pulley of cable column 7 is attached,and on the other end of which a pinion gear is formed. As a steeringangle sensor 11, an absolute type resolver or the like, which detectsthe rotational velocity of the shaft can be used. Also, torque sensors3, and torque sensors 12 form a double system that detects torque fromchanges in inductance. Steering angle sensor 11 is set on the side ofcable column 7, and torque sensors 12 are set on the side of steeringunit 15. As a result, when the steering angle is detected by steeringangle sensor 11, it is unaffected by the change in the angle due to thetwisting of torque sensors 12.

The steering motors 14 have a structure in which a pinion gear engagedto the worm gear set at the central position between steering anglesensor 11 of the pinion shaft 17 and torque sensors 12 is set on themotor shaft, so that a steering torque is applied to pinion shaft 17when the motor is on. The steering motors 14 form a double system with a1-rotor/2-stator structure. The steering motors 14 are brushless motorsthat form first and second steering motors 14. Similarly, in thereaction force motor 5, since brushless motors are used, encoder 10 anda Hall IC (not shown in the figure) are used.

The steering unit 15 has a structure in which left/right steered roadwheels 16, 16′ turn as pinion shaft 17 rotates. It has rack shaft 15 bthat forms a rack gear engaged with the pinion gear of pinion shaft 17and inserted in rack tube 15 a, tie rods 15 c, 15 c′ fixed to the twoends of rack shaft 15 b extending in the left/right direction of thevehicle, and knuckle arms 15 d, 15 d′ having one end fixed to the tierods 15 c, 15 c′ and the other end fixed to the steered road wheels 16,16′.

The controller has a double system design composed of two power sources18, 18′ and two controllers 19, 19′ that perform processing andarithmetic operations.

The controllers 19, 19′ receive the detected signals from the followingparts: steering angle sensor 1, encoder 2, torque sensors 3, and theHall IC of the reaction force device, as well as encoder 10, steeringangle sensor 11, torque sensors 12, Hall IC, and vehicle speed sensor 21(vehicle speed detection means) of the steering device.

On the basis of the detection values of the various sensors, controller19 sets the control quantities of reaction force motor 5 and steeringmotor 14, and controls and drives each of steering motors 14. Also,during ordinary system conditions, controller 19 releases the clutch 9.Otherwise, the system engages clutch 9 to establish a mechanicalconnection between steering wheel 6 and the steered road wheels 16, 16′.

By means of steering motor 14, the following formula I is used to setcontrol value (Th) of reaction force motor 5 for computing the reactionforce motor control valve.Th=Kp×θ+Gf×F  (1)

Here, (Kp) represents the steering angle feedback gain, θ represents thesteering angle, (Gf) represents the road surface reaction force feedbackgain, (F) represents the road surface reaction force. The first term onthe right-hand side sets the control value of the steering reactionforce on the basis of steering angle θ, and the second term on theright-hand side sets the control value on the basis of road surfacereaction force (F), so that it can reflect the influence on the steeringreaction force of the force on the tires from the road surface on thetires.

Here, road surface reaction force feedback gain (Gf) changes as afunction of the steering state. Its value is set such that in the caseof turning the steering wheel, the road surface feel is transmitted tothe driver through an appropriate steering reaction force. The amount offeedback of the road surface reaction force component is set smaller sothat during the steering wheel return operation, the steering wheel isnot hindered by excessive shock forces, etc.

The road surface reaction force feedback gain (Gf) is as follows:

-   -   Gf(low)=(Low) . . . during the steering wheel return operation    -   Gf(High)=(High) . . . during the steering wheel turning        operation        The (Low), (High) are determined on the basis of the graph to be        explained below (FIG. 4). However, they may also be preset        constants.

Also, on the right-hand side of formula (I), it is possible to set thecontrol value on the basis of steering angle velocity dθ/dt and steeringangle acceleration d²θ/dt². In this case, control value (Th) of reactionforce motor (4) is determined according to Equation 2 below.Th=Kp×θ+Kd×dθ/dt+Kdd×d ² θ/dt ² +Gf×F  (2)Here, Kd and Kdd are preset constants.

Setting of the road surface reaction force feedback gain is discussedhereinafter. FIG. 3 is a flow chart illustrating the method for settingroad surface reaction force feedback gain (Gf). Each step will beexplained below.

In step S1, the signal from steering angle sensor 1 is read, and processcontrol then goes to step S2.

In step S2, from the sensor signal read in step S1, the steering angleand steering angle velocity are computed (corresponding to the steeringangle velocity detection means), and it is determined whether thesteering wheel is in the return state (corresponding to the turn/returnjudgment means). If YES, control goes to step S3, and if NO, it goes tostep S4.

In step S3, the road surface reaction force feedback gain (Gf) is setLow (corresponding to the steering reaction force correction means), andit returns.

In step S4, road surface reaction force feedback gain (Gf) is set toHigh, and it returns.

FIG. 4 is a graph used to set road surface reaction force feedback gain(Gf) corresponding to road surface reaction force F. In this case ofsteering wheel return (Low), compared with the case of steering wheelturn (High), road surface reaction force feedback gain (Gf) is set to asmaller value with respect to road surface reaction force F.

Setting the control value corresponding to the steering state isdiscussed hereinafter. When it is necessary to execute quick return ofthe steering wheel, in order to ensure that the steering wheel returnoperation of the driver is not hindered in the first embodiment, as thesteering angle velocity dθ/dt increases, the amount of feedback from theroad surface reaction force decreases.

Road surface reaction force feedback gain (Gf) becomes the following:

-   -   Gf(low)=(Low)×L1 . . . . During the steering wheel return        operation        -   Gf(High)=(high) . . . during the steering wheel turn            Operation

Here, (L1) is set on the basis of the graph shown in FIG. 5. In thegraph shown in FIG. 5, (L1) has a maximum value of 1 in the range ofhigh frequency of occurrence generated for steering angle velocity dθ/dtin the case of high vehicle speed, and the value decreases as thesteering angle velocity dθ/dt rises. When the steering angle velocitydθ/dt reaches the region of emergency avoidance maneuvers, the value L1reaches the maintain value, L1min.

Setting the control value corresponding to the vehicle speed isdiscussed hereinafter. In the first embodiment, because there are moreinstances of quick steering wheel return when vehicle speed (V)decreases, the road surface reaction force amount of feedback in thesteering wheel return operation is set smaller in the low velocityregion. That is, the higher the speed, the more sensitive the vehiclebehavior with respect to steering wheel maneuvering, so that even in thesteering wheel return operation, road surface reaction force feedback isstill required. On the other hand, in the turn area of the low speedregion, smooth steering wheel maneuverability is required. Consequently,in light of this fact, the amount of feedback for the road surfacereaction force is set to be smaller in the case of steering wheel returnwhen the vehicle speed (V) is lower.

Consequently, road surface reaction force feedback gain (Gf) is asfollows:

-   -   Gf(Low)=(Low)×L1×L2 . . . during the steering wheel return        operation        -   Gf(High)=(High) . . . during the steering wheel turning            operation

Here, (L2) is set on the basis of the graph shown in FIG. 6. In thegraph shown in FIG. 6, (L2) rises in proportion to vehicle speed V. Itis set such that it reaches the maximum value 1 when vehicle speed (V)reaches the high frequency of occurrence region when the vehicle is in ahigh-traffic area.

In the steering reaction force computation unit, the average value ofthe detection results (F1), (F2) of the steering reaction force sensorsset at the two ends of the steering rack is taken as steering reactionforce (F) is applied to the steering shaft (pinion shaft). In thesteering shaft motor, on the basis of these computational results,rotation control value (Mm) of the steering shaft is computed using thefollowing Equation 3, and the reaction force control signalcorresponding to rotation control value Mm is output to steering shaftmotor.Mm=Gm×(aT−F)  (3)Here, (Gm) represents the gain coefficient indicating the gain of theoutput signal.

In the first embodiment, because road surface reaction force feedbackgain (Gf) is made smaller in the steering wheel return operation in thesteering device, even if the road surface reaction force risestransiently due to the rough road surface, changes in the steering forceaccompanying the shock are suppressed. Therefore, the problem ofhindering quick maneuvering of the steering wheel can be alleviated(FIG. 8), so that the driver can return back the steering wheel to thecenter position smoothly.

Also, when steering angle velocity dθ/dt is higher, road surfacereaction force feedback gain (Gf) becomes smaller. Consequently, in thecase of quick return steering, by suppressing changes in the steeringforce accompanying shock can be suppressed, and quicker return steeringby the driver is not hindered. In addition, when vehicle speed (V) islower, road surface reaction force feedback gain (Gf) becomes smaller.Consequently, a good maneuvering of the steering wheel in the low-speedregion and a high vehicular travel stability in the high-speed regioncan be realized at the same time.

The effects of the vehicle steering device of the first embodiment willnow be discussed. In the vehicle steering device, the steering wheel 6,which receives the steering input, and the steering unit 15, whichsteers the steered road wheels 16, 16′, are mechanically separated.Corresponding to road surface reaction force F, steering reaction forcecorrection (Gf×F) is added to steering wheel 6. The vehicle steeringdevice includes the following parts: turn/return judgment means thatjudges turn/return of steering wheel 6, and a steering reaction forcecorrection means that has a smaller road surface reaction force feedbackgain (Gf) during return of steering wheel 1 than during its initialturning. It is possible to suppress the change in the steering forceaccompanying the shock from the road, and it is possible to reduce theprobability of an ineffective steering wheel.

Because the steering reaction force correction means has a smaller roadsurface reaction force feedback gain (Gf) for a higher steering anglevelocity dθ/dt of steering wheel 1, it is possible to ensure that aneven quicker return of the steering wheel 6 by the driver is nothindered.

Because the steering reaction force correction means has a smaller roadsurface reaction force feedback gain (Gf) for a lower vehicle speed V,it is possible both to feed back the road surface reaction force in thehigh-speed region and to improve the steering wheel return operation inthe low-speed region.

In the second embodiment, the road surface reaction force amount offeedback is changed corresponding to the vehicle state value. Thestructure of the second embodiment is the same as that of the firstembodiment shown in FIGS. 1 and 2, so that its explanation will not berepeated.

The control value is set corresponding to the vehicle state value. Ifthe amount of feedback of the road surface reaction force in thesteering wheel return operation is set to a small value, the overallsteering reaction force may become too small. In this case, as thevehicle state value, the yaw rate is computed (corresponding to thevehicle state value detection means). It also computes gain constant(Gy) corresponding to the amount of feedback of the road surfacereaction force or lower and reduction component (YD) of the amount offeedback of the road surface reaction force. By adding it to the controlvalue of reaction force motor (4), it is possible to prevent thesteering reaction force from becoming too small.

In the second embodiment, on the right-hand side of Equation 2, thecontrol value is set and added on the basis of yaw rate ψ whichindicates the behavior of the vehicle. Consequently, control value (Th)of reaction force motor (4) can be described by Equation 4 below.Th=Kp×θ+Kd×dθ/dt+Kdd×d ^(2θ) /dt ² +Ky×ψ+Gf×F  (4)

The method for computing the yaw rate is discussed hereinafter. The yawrate ψ can be obtained using the Equation 5 below from steering angle θand vehicle speed (V) by means of a mathematical operation on thevehicle movement.ψ={G×ωn ² ×Tr(s+1/Tr)×θ}/(s ²+2ξωns+ω ²)  (5)

-   -   G={1/(1+A×V²)}×(V/L)    -   Tr=(2 Lr×Kr)/(m×Lf×V)    -   A=(m/2L²)×{(Lf×Kf−Lr×Kr)×(Kf×Kr)}

Here, Lf represents the distance between the center of gravity and thefront shaft, Lr represents the distance between the center of gravityand the rear shaft, Kf represents the cornering force of the frontwheels, Kr represents the cornering force of the rear wheels, mrepresents the weight of the vehicle, and s represents the Laplaceoperator.

Consequently, the value obtained using the Equation 5 is used as theestimated value of yaw rate ψ.

Using vehicle speed (V), there is the following relationship of Equation6 between lateral acceleration (Yg) and yaw rate ψ.Yg=ψ×V  (6)

Also, as far as road surface reaction force (F) is concerned, whensteady-state circular rotation free of external disturbance isperformed, the relationship of the following Equation 7 is established.F∝Yg  (7)

Consequently, one has the following relationship:F∝ψ×V  (7)′

For a decrease (YD) of the amount of feedback of the road surfacereaction force in the steering wheel return operation, from the graphshown in FIG. 9, the High′ value and Low′ value of (Tf) are readcorresponding to ψ×(V), and it becomes the following Equation 8(corresponding to the steering reaction force correction valueestimation means).YD=Tf×High′−TfLow′  (8)The value obtained by multiplying preset gain constant Gy by (YD) isadded to control value (Th) of reaction force motor (5).

Here, (Gy)=(Gf) (FIG. 10).

However, the present invention is not limited to this scheme. Forexample, one may adopt Gy=AGf, where A=1 in the high-speed region and asmaller value of (A) as the speed decreases.

Also, as stated above, for (YD), one may haveYD=Tf×High′−Tf×Low′×L1×L2  (8)′

FIG. 11 is a diagram illustrating the return steering operation of thesecond embodiment (solid lines) as compared to the first embodiment(dotted lines). Unlike the first embodiment, (YD) corresponding to theamount of feedback of the road surface reaction force is added to thesteering force in the second embodiment. Consequently, when the roadsurface reaction force changes without influence of the lateralacceleration and yaw, it is possible to reduce the change in thesteering force accompanying the shock, while preventing the steeringforce from becoming too small.

In the following, an explanation is given regarding the effects for thevehicle steering device of the second embodiment. In addition to effectsas discussed regarding the first embodiment, the second embodiment alsoprovides the following effect, in that the second embodiment of theinvention has a steering reaction force correction value estimationmeans that estimates (YD) corresponding to the decrease in the amount offeedback of the road surface reaction force from yaw rate ψ. Thesteering reaction force correction means adds the steering reactionforce component (Gy×YD) corresponding to yaw rate ψ. Consequently, itcan reduce the change in the steering force, and it can prevent thesteering force from becoming too small.

The third embodiment is an example of correcting the amount of feedbackof the road surface reaction force when the vehicle is within therotation limit region. In this case, the rotation limit region refers tothe state in which lateral tire skidding takes place. Also, since thestructure of the third embodiment is the same as that of the firstembodiment shown in FIG. 1, it will not be explained in detail again.

In the following, an explanation is given regarding the operation of thethird embodiment setting the road surface reaction force feedback gainis discussed hereinafter. FIG. 12 is a flow chart illustrating themethod for setting road surface reaction force feedback gain (Gf) of thethird embodiment. In the following, an explanation will be givenregarding the various processing steps.

In step S11, the signals from steering angle sensor 11 and vehicle speedsensor 21 are read, and process control goes to step S12.

In step S12, from the steering angle of steered wheels 16, 16, vehiclespeed V, and yaw rate Ψ of the vehicle read from step S11, it isdetermined whether the vehicle is within the rotation limit region withreference to the graph shown in FIG. 13 (corresponding to the rotationlimit judgment means). If YES, it goes to step S13, and, if NO, it goesto step S14. In step S13, road surface reaction force feedback gain (Gf)is fixed at (High), and then process control returns. In step S14,control is continued by varying road surface reaction force feedbackgain (Gf) in the steering wheel turn/return operation, and then itreturns.

Return steering operation of the third embodiment will now be discussed.When the vehicle is within the rotation limit region, the drivercorrects the vehicle behavior by means of correction steering. In thiscase, turn/return of steering wheel 6 is performed at high frequency insmall increments/decrements. In this case, the scheme in transferringthe amount of feedback of the road surface reaction force to the driverallows the driver to correct the vehicle behavior more easily.

Consequently, in the third embodiment, when the vehicle is within therotation limit region, the correction control of the amount of feedbackof the road surface reaction force is released. Consequently, thesteering reaction force corresponding to the vehicle behavior can betransmitted to the operator. Consequently, accompanying the decrease inthe amount of feedback of the road surface reaction force, the steeringcorrection by the operator is not hindered.

In the third embodiment, in addition to the effects as discussedregarding the first embodiment, vehicle steering device has thefollowing additional effect in that there is a rotation limit regionjudgment means that determines whether the vehicle is at the rotationlimit on the basis of the steering angle of steered wheels 16, 16′ andthe yaw rate v of the vehicle. When it is judged that the vehicle is atthe rotation limit, the steering reaction force correction means doesnot reduce the amount of the steering reaction force correction, so thatby reducing the amount of feedback of the road surface reaction force,the steering correction operation by the driver is not hindered.

The above-mentioned embodiments have been described in order to alloweasy understanding of the present invention. The invention is not to belimited to the disclosed embodiments but, on the contrary, is intendedto cover various modifications and equivalent arrangements includedwithin the spirit and scope of the appended claims, which scope is to beaccorded the broadest interpretation so as to encompass all suchmodifications and equivalent structures as is permitted under the law.

For example, one may also use the lateral acceleration as the vehiclestate value. However, since the yaw rate usually changes faster than thelateral velocity, from the standpoint of system response, it ispreferred that the yaw rate be used. Also, in the first embodiment, theyaw rate is computed using Equation 1. However, it is also possible touse the detection value from a yaw rate sensor. Also, a controller caninclude control function distributed among multiple processors.

This application is based on Japanese Patent Application No.2004-350371, filed Dec. 2, 2004 in the Japanese Patent Office, theentire contents of which are hereby incorporated by reference.

1. A steering control device for use in a vehicle having a steeringwheel that receives steering input, and an electronically-controlledsteering unit that turns the vehicle's wheels over a road surface basedon the position of the steering wheel, comprising: a reaction forcedevice coupled to the steering wheel and responsive to a control signalto apply a steering reaction force to the steering wheel; and acontroller configured to generate the control signal in response to themovement of the steering wheel and the road surface reaction force,wherein the controller is configured to vary the control signal toincrease the steering reaction force in response to the road surfacereaction force, and to determine that the reaction force is larger whenthe steering wheel is turning than when the steering wheel is returning.2. The steering control device of claim 1, wherein the controller isresponsive to the angular velocity of the steering wheel, and is furtherconfigured to vary the control signal as a function of steering wheelangular velocity.
 3. The steering control device of claim 2, wherein thecontroller is further configured to vary the control signal to decreasethe steering reaction force at higher steering angular velocities. 4.The vehicle steering control device of claim 1, further comprisingvehicle speed sensor wherein the controller is responsive to the vehiclespeed sensor and is further configured to vary the control signal as afunction of vehicle speed.
 5. The steering control device of claim 3,wherein the controller is configured to vary the control signal todecrease the reaction force at lower vehicle speed.
 6. The steeringcontrol device of claim 1, wherein the controller is further configuredto calculate a vehicle state and an estimated steering force correctionvalue based on the vehicle state, and to vary the control signal as afunction of the estimated steering force correction value.
 7. Thesteering control device of claim 6, wherein the controller is configuredto vary the control signal to increase the steering reaction force bythe steering force correction value.
 8. The steering control device ofclaim 6, wherein the vehicle state is calculated based on information ofthe vehicle speed and yaw.
 9. The vehicle steering control device ofclaim 1 wherein the controller is further configured to calculatewhether the vehicle is within a rotation limit based on the angle of thewheels and the yaw rate of the vehicle, and wherein the controller isconfigured to not reduce the steering reaction force when the controllerdetermines that the vehicle is within the rotation limit.
 10. A vehicle,comprising: (a) a steering wheel that receives steering input from anoperator; (b) an electronically controlled-steering device that turnsthe vehicle's wheels over a road surface based on the position of thesteering wheel; (c) a reaction force device coupled to the steeringwheel and responsive to a control signal to apply a steering reactionforce to the steering wheel; and (d) and a controller configured togenerate the control signal in response to the movement of the steeringwheel and a road surface reaction force, wherein the controller isconfigured to vary the control signal to increase the steering reactionforce in response to the road surface force when the steering wheel isturning and to decrease the steering reaction force in response to theroad surface force when the steering wheel is returning.
 11. Thesteering control device of claim 10, wherein the controller isresponsive to the angular velocity of the steering wheel, and is furtherconfigured to vary the control signal as a function of steering wheelangular velocity.
 12. The steering control device of claim 11, whereinthe controller is further configured to vary the control signal todecrease the reaction force at higher steering angular velocities. 13.The vehicle steering control device of claim 10, further comprisingvehicle speed sensor wherein the controller is responsive to the vehiclespeed sensor and is further configured to vary the control signal as afunction of vehicle speed.
 14. The steering control device of claim 13,wherein the controller is configured to vary the control signal todecrease the reaction force at lower vehicle speed.
 15. The steeringcontrol device of claim 10, wherein the controller is further configuredto calculate a vehicle state and to estimate a steering force correctionvalue based on the vehicle state, wherein the controller is configuredto vary the control signal as a function of the estimated steering forcecorrection value.
 16. The steering control device of claim 15, whereinthe controller is further configured to vary the control signal toincrease the reaction force by the steering force correction value. 17.The vehicle steering control device of claim 10, wherein the controlleris further configured to calculate whether the vehicle is within arotation limit based on the angle of the wheels and the yaw rate of thevehicle, and to not reduce the steering reaction force when thecontroller determines that the is within the rotation limit.
 18. Ansteering control apparatus for use in a vehicle having a steering wheeland a reaction device to impose a steering reaction force onto thesteering wheel in response to a steering force control signal,comprising: steering force correction means for calculating the steeringforce control signal based on a road surface reaction force and a gain;judgment means for determining whether the steering wheel is in aturning or returning mode; and control means for setting the gain at ahigher value when the steering wheel is in a turning mode.
 19. A methodfor controlling steering in a vehicle having a steering wheel and areaction device to impose a steering reaction force onto the steeringwheel in response to a steering force control signal, comprising:calculating the steering force control signal based on a road surfacereaction force and a gain; determining whether the steering wheel is ina turning or returning mode; and setting the gain at a higher value whenthe steering wheel is in a turning mode.
 20. The method of claim 19,further comprising: determining the angular velocity of the steeringwheel; and setting the gain at a lower value when the steering wheel hasa higher angular velocity.
 21. The method of claim 19, furthercomprising: determining the vehicle speed; and setting the gain at alower value when vehicle speed is lower.
 22. The method of claim 19,further comprising: calculating a vehicle state value; and estimating asteering reaction force correction value from the vehicle state value;and adding the steering reaction force correction value to the steeringforce control signal.
 23. The method of claim 19, further comprising:determining whether the vehicle is within a rotation limit; and settingthe gain to a predetermined high value when the vehicle is within therotation limit.